When producing powders, especially metallic powders, there is often an unintentional oxidation of the surfaces of the powders during production. Furthermore, oxygen might be present inside the powder itself, either in solution or as oxide particles. In this latter case the oxygen is usually generated during the melting process due to equilibrium with the dross and the lining of the furnace.
The oxides, especially the oxides of the powder surfaces, might lead to deteriorated mechanical properties of a component produced to near-net-shape (NNS) of a powder by densification. In the case of surface oxides, a network of oxide inclusions will form where the surfaces of the powder were located before densification.
One example of a powder that suffers from the above stated problems is powder of super duplex stainless steels (SDSS). Dense bodies of SDSS can be used in various different environments. One application is in the oil and gas industry. However, dense bodies of SDSS produced by powder metallurgy generally suffer from low impact strength. One theory of the reason for this problem is that intermetallics precipitate at oxide inclusions. Another theory is that intermetallics and oxide precipitates both decrease the impact strength, however separately. In either case, there is a need of reduced oxygen content of the powder.
However, even other powder materials, such as metallic powders or hard materials, might have a too high content of oxygen to achieve good mechanical strength, such as impact strength, after compacted to a dense body. This is especially important for materials that easily oxidise during powder formation even if precautionary measurements have been taken.
It is previously known to utilise a getter to minimise the oxygen content when producing dense products by powder metallurgy technique. For example, U.S. Pat. No. 3,992,200 discloses the use of a getter consisting of Ti, Zr, Hf and mixtures thereof to prevent oxide formation in the final compacted article. This method is for example utilised on high-speed steels and superalloys. Furthermore, U.S. Pat. No. 6,328,927 discloses the use of a getter when manufacturing dense bodies of tungsten. In this case the powder capsule is made of the getter material, such as titanium or alloys thereof.
However, merely utilising a getter material does not sufficiently reduce the oxygen content to the desired low levels of all powders, especially of all powders of steels. This is especially difficult in powders wherein the carbon content is low, such as ≦0.1%. The time for reduction, and hence the result, is difficult to accomplish in a controlled manner and in a cost-effective way.
Consequently, there is a need for a method of reducing the oxygen content of a powder in a controlled manner before densification, especially for low oxygen contents.
Also, there is a need for reducing the oxygen content of low carbon steels, having a high Cr content, to very low levels, such as less than 100 ppm.